Strange 555 failure

[throbscottle]

Just having to replace a 555 timer where it looks like the middle one of it's 3 internal resistors has failed S/C, or something else has blown between the 2 comparators. It was fairly new too - genuine TI bought from RS Components some years ago. Now has nearly the same resistance between it's CV pin and Vcc or Gnd.

It's being used as astable to produce a PWM signal (around 2KHz) with 5v supply. The only action being seen by the CV pin is to be connected periodically (period of several seconds) to 0v by a transistor. It's currently also got a decoupling cap attached to it but I might take that out.

Do you think something could have caused this to happen, or have I just been unlucky?

 

[aliarifat794]

Was the 555 used before?

 

[throbscottle]

No, it was brand new out of the AS bag Unusual for me...

 

[alec_t]

Now has nearly the same resistance between it's CV pin and Vcc or Gnd.

What value resistance?

 

[schmitt trigger]

Resistors on ICs ARE NOT your discrete resistors. They are isolated from the substrate and everything else with diffused isolation wells, that REQUIRES to be biased to perform its function.

If you are attempting to measure internal resistance on an unpowered IC, most likely you are measuring leakage from the unbiased wells.

Additionally, in modern ICs the voltage divider function may use Vbe multipliers or other techniques that lend themselves better to IC integration instead of actual resistances.

 

[throbscottle]

Out of circuit, approximately 4.9k each way. Might have been lower, I can't check right now. But I think my meter reads a little low. They were very similar values anyway. Another new one out of the bag gave what you'd expect, 4.9k to Vcc and 9.8k to Gnd.

Schmitt trigger - point taken. But on the 555 they do look like normal resistors from a measurement point of view. But that brings me back to my original thought that something else had failed causing the short. Could the isolation well have failed? In which case what might have caused it? Accidental charge pump?

 

[Ramussons]

Put it in the circuit and if it does not function, then suspect it. IC's cannot be tested by measuring resistances between its pins.

 

[throbscottle]

Ramussons, if you had read the first post you would know that the chip is already a known failure, where the 3 resistor divider has become effectively a 2 resistor divider (which showed up as the CV pin being at Vcc/2, although I didn't mention that originally). I've already removed and replaced it and there is no problem with the replacement. The comparative resistance measurements with a known good chip are subsequent to that.

I'm just trying to do something of a post-mortem to work out why it happened, since the chip was new and the circuit hadn't been run very much, and it seems like such an odd way to fail.

 

[eTech]

the chip is already a known failure,where the 3 resistor divider has become effectively a 2 resistor divider (which showed up as the CV pin being at Vcc/2, although I didn't mention that originally).

My guess is that one of the transistors in the trigger circuit failed, causing a short across the last resistor (the one connected to ground), of the internal resistor divider. The resistor traces should be large relative to the others, i don't think they would fail.

 

[throbscottle]

Hmm, I was thinking a failure connecting the threshold and trigger reference inputs together. But your idea seems more likely.
The chip does partially work, the astable was running, just with the wrong frequency/duty, and limited range in the control. I suppose if I see what voltage the trigger input will work at it will verify or disprove your theory. I know it all looked pretty low when I was tracing the fault.

 

[crutschow]

Measure the voltage at the CV pin with power applied.
It should be about 2/3rds the supply voltage.

 

[throbscottle]

Mentioned in post #8

 

[schmitt trigger]

As part of a reliability group which helped investigate field failures, I did my fair share of IC post mortems. An IC requires decapsulation to observe the chip itself, which is a dangerous operation, and requires intimate knowledge of hazardous materials handling.
Next you require a top of the line optical microscope to find the general area of the failure, and then a scanning electron microscope to actually view the failure itself. Then you have to interpret the actual images because the failures almost always resemble microscopic volcanoes.

It is a very specialized process.

 

[throbscottle]

Okaaaayyy... I don't have that equipment or training unfortunately. It would be very interesting though. For me it's more a case of "is it something I did?" - If I can work out what failed it gives more of an answer to that question. But it's looking like, no, I was just unlucky.

I have managed to remove the covering from a 40 pin COB chip I replaced (with a 44 pin one, that was fun!) in the process of removing the whole thing it was a glass fibre pencil that seemed the most effective tool and cleanly exposed the silicon - I might give that a try. OTOH the chip still partially works so I might be able to use it for /something/...

Maybe try it on something else. I have a bagful of useless board-pulls to go at! (See what my x20 magnifier can show me. Not quite a SEM I know...)